1. Scope of the Present Invention
The present invention is directed to the method and apparatus for the vacuum distillation and recovery of solvents used in industrial processes. In particular, the present invention embodies an automated system wherein large quantities of solvent can be processed and recovered either in a continuous or batch-wise operation.
2. Description of the Prior Art
The process for the distillation of solvents for purposes of recovering solvents has been known in the art for many years. There are possibly dozens of companies world wide who manufacture the equipment intended for this use. However, most such companies, if not all, employ the same technology, or combinations of technologies currently used in distillation to implement their particular implementations or applications of the general process.
Portable solvent recovery machines generally incorporate a vessel, usually thermally insulated for efficiency, with a heating element or heating source of some type (e.g., an electric heating element). Most such vessels are cylindrical in shape and house the electric heating element immersed in a bath of thermal oil for even, and most important, safe heat transfer. Variations of such machines include those with rectangular vessels or encapsulated heating elements, which eliminates the need for thermal oil. There are also machines that use steam produced in-house and piped through tube bundles for heating the solvent to its boiling point.
In a typical operation, when the vessel heats the solvent to its boiling point, the vessel allows the solvent vapor to exit, thus leaving contaminants behind. After leaving the boiling vessel, the solvent vapor then passes through a condenser or heat exchanger to change the solvent vapor back into a liquid. Such condensers or heat exchangers range from simple water jackets to refrigerated shell and tube bundles. The medium used for removing the heat from the condensers is usually water. The source varies from ordinary tap water lines, cooling tower lines, or condensing water lines.
Some portable distillation devices utilize vacuum systems to assist in distillation. Depending on the type and performance of the vacuum system, it can lower the boiling point of most solvents by 100 degrees Fahrenheit or more. Most solvent recovery units on the market today that incorporate vacuum systems actually create a negative pressure on a self-contained process. This means that, once an operator fills the solvent recovery system with dirty solvent, the machine then creates a vacuum on the entire process until all of the solvent is refined. After this is accomplished, the operator usually stops the machine, breaks the vacuum, drains or pumps out the clean solvent, refills the unit with dirty solvent and starts again. This is typically referred to as a vacuum assisted "batch process."
More sophisticated vacuum assisted distillation units offer a "continuous process" feature. In such units, the vacuum system maintains a negative pressure on the vessel and vapor side and a positive pressure on the newly condensed clean solvent line. This allows the operator to do hundreds of gallons of solvent without interruption or down time. This can be achieved by using a jet-pump type system or a liquid ring vacuum system.
The methods used in controlling and monitoring the various aspects of conventional machines, as described above, are usually very similar, although there are some slight variations. Many of the older models use a mechanical thermostat built into the heating element to control the upper limit of the heating oil. By setting an upper limit on the heating oil, the operator can distill a selected solvent at temperatures well below auto-ignition. Systems incorporating this simplified technology usually rely on a timer or frequent operator attention in order to detect when production has ceased. Timers as used in the art have been found to rarely provide accurate predictions of process completions.
Different types and quantities of contaminants will vary the length of the distillation cycle for any given solvent. Typically, the operator has to perform numerous trial runs with varying levels of solid contents to establish a smooth routine. Incorrect cycle timings can, however, cause premature inner tank inspections resulting in potential VOC emissions, or at best, require annoyingly difficult still-bottom removal operations.
Applicant has found that measuring and controlling the heating oil temperature, in conjunction with measuring and controlling the solvent vapor temperature is a far better technique for controlling and monitoring the overall solvent distillation and recovery operation. The importance of being able to read and control the vapor temperature cannot be overstated. Applicant has further found that increases and decreases of vapor and oil temperatures do not always parallel each other. When a distillation cycle begins, the temperatures of the heating oil and the dirty solvent begin to rise uniformly. The vapor temperature remains relatively ambient, disregarding moderate conditional heat transfer, until initial signs of clean solvent production appear.
As the production of the clean solvent progresses, the vapor temperature quickly responds, climbing in a manner proportionately similar to the rise in heating oil temperature. During a "continuous process" or during initial stages of a "batch process," vapor temperatures can easily be controlled by limits set on the heating system. However, during the final stages of either a "batch" or "continuous" process, vapor temperatures begin to change disproportionately relative to the heating system.
As the temperature of the solvent vapor in the heating vessel drops significantly, the amount or quantity of solvent vapor, or its product, decreases as well. When this occurs, the distillation system's ability to remove the heat through vaporization is impaired, thus resulting in dramatic increases in vapor temperatures, even though the upper limits of the heating system are satisfied.
In applications where precise sensing is used in distilling solvents that contain nitrocellulose or chlorine, the existence of uncontrollable vapor temperatures can be very critical. Nitrocellulose solvents, commonly used in the printing industry, auto-ignite at 235.degree. Fahrenheit, and therefore must be accurately monitored during recovery. Most chlorinated solvents used in the U.S. dry-cleaning industry have been carefully formulated to allow safe use and safe reclamation, if distilled properly. However, the reclamation of those chlorinated solvents require that the vapor temperatures be closely regulated to insure that destructively high temperatures do not occur. If such temperatures were to occur, the integrity of the stabilization process done during the formulation of the chlorinated solvent can be compromised. As a result, free chlorine molecules that may be harmful to people and/or the atmosphere can be generated. Excessively high vapor temperatures can also damage the formulations of "blended" solvents, such as Varsol.RTM., resulting in solvents which are no longer suitable for their intended purpose.
A major problem with distillation units today is operator intervention. Too many units require frequent operator intervention to insure that the distillation process is performing properly. This is due mainly to the equipment's inability to perform control logic, or reason. Applicant has found that on-board programmable logic controllers (PLCs) or even microprocessors, in conjunction with proper temperature and pressure sensing, would greatly reduce or eliminate this problem. However, in order to achieve closed-loop solvent distillation, units incorporating automated control systems would need to control and/or monitor dirty solvent availability, filter cleanliness, automatic shut down, system diagnostics, and continuous feed capabilities.
In other applications of solvent distillation and recovery processes, government safety and/or environmental regulations require that the hydrocarbons in the still-bottom residues be removed in order to render the contaminants landfill ready if the contaminants themselves are to be classified as nonhazardous. On the other hand, residues that are classified as hazardous may require some fluidity for optimum removal or reasonable BTU values for economic incineration.
As shown above, there exist numerous considerations in the design of the structure and operation of solvent distillation and recovery systems. In addition to such design considerations as those discussed above, further considerations include the incorporation of energy efficient heating components, substantial insulation properties, proper heat exchange equipment and sizing, quality component materials and multiple safety back up systems, and the elimination of dangerously hot components exposed to the operator. Unfortunately, most manufacturers of conventional solvent distillation and recovery systems do not conform to all of these essential requirements.
Consequently, there exists a need for a solvent distillation and recovery system that does incorporate features based on all of the above considerations. In particular, there exists a need for an automated solvent distillation and recovery system that allows the user to selectively operate the system according to the specific requirements of the particular solvent and contaminants being processed.
At the same time, there exists a need for a solvent distillation and recovery system that can be operated in either a continuous or "batch-type" process, that monitors and controls the heating element and solvent vapor temperatures, and that can accommodate the specific requirements for processing different solvents and different contaminants.
Further, there exists a need for a solvent distillation recovery system that incorporates energy efficient heating components, exhibits effective insulation properties, utilizes effective properly sized heat exchange equipment, uses quality component materials and multiple safety back up systems, and isolates the operator from dangerously hot components.